1. Field of the Invention
This invention relates to an acceleration detector and more particularly to an acceleration detector for detecting knocking in an internal combustion engine.
2. Description of the Related Art
FIGS. 1 and 2 illustrate one example of an acceleration detector to which the present invention can be applied. The acceleration detector comprises a housing 1 defining an annular cavity 2 therein and an annular acceleration transducer assembly 3 disposed within the cavity 2. The housing 1 comprises a tubular, electrically conductive metallic bushing 4 having a through hole 5 and a flange 6. The housing 1 also comprises a ring-shaped resinous outer case 7 bonded by a bonding agent 7a to the flange 6 of the bushing 4 so that the cavity 2 is defined therein.
The outer case 7 also has a connector 8 radially outwardly extending from the outer case 7 so that an external output terminal 9 can extend through the connector 8 for taking out an output signal from the acceleration transducer assembly 3 disposed within the cavity 2. The acceleration transducer assembly 3 further includes an annular piezoelectric element 11 placed on the terminal plate 10, a washer-shaped output terminal 12 including a lead 12a connected to the external output terminal 9, an electrically insulating washer 13 disposed on the output terminal 12, an annular inertial weight 14 placed on the insulating washer 13 and a threaded ring-shaped stop nut 15 thread-engaged with the thread 4a on the tubular bushing 4. An electrically insulating tape or tube 16 is placed on the tubular bushing 4 so that the acceleration transducer assembly 3 is insulated from the bushing 4 even when the output terminal 12 as well as the piezoelectric element 11 are eccentrically assembled.
In order to resiliently support and protect the acceleration transducer assembly 3 within the cavity 2 from undesirable environmental conditions, the remaining space of the cavity 2 of the housing 1 which is not occupied by the acceleration transducer assembly 3 is substantially filled with a resilient filler material 17 of a thermo-setting resin. The filler material 17 must be sufficiently resilient after it is cured to allow the movement of the inertial weight 14 relative to the housing 1 when an acceleration is applied to the inertial weight 14 so that the piezoelectric element 11 generates a voltage signal proportional to the pressure exerted on it by the relative movement of the inertial weight 14 against the piezoelectric element 11.
When in use, the acceleration detector is securely mounted on the internal combustion engine (not shown) by a suitable bolt (not shown) inserted into the central through hole 5 of the housing 1. The acceleration or the vibration of the internal combustion engine produces the movement of the inertial weight 14 relative to the housing 1, which causes the piezoelectric element 11 to be stressed by the inertial weight 14, whereby an electrical signal indicative of the movement of the inertial weight 14 relative to the engine is generated from the piezoelectric element 11. The electrical signal is provided from the output terminal 9 to be analyzed to determine as to whether or not a knocking signal which generates upon knocking of the internal combustion engine is involved. When it is determined that a knocking signal is contained in the electrical signal, the operating parameters for operating the engine can be adjusted to increase the output power or decrease the fuel consumption rate.
In the conventional acceleration detector as above described, since the washer-shaped output terminal 12 is inserted between the piezoelectric element 11 and the insulating washer 13, a parallel electrostatic capacitance circuit as illustrated in FIG. 3 is established between an electrostatic capacitance C11 of the piezoelectric element 11 and an electrostatic capacitance C13 of the insulating washer 13. As seen from FIG. 3, the overall capacitance C of the detector is the sum C11+C13 of the parallel-connected capacitances and is not determined by the capacitance C11 alone, but is relatively greatly affected by the capacitance C13 of the insulating washer 13. When the insulating washer 13 is made of polyester, for example, the capacitance C13 of the insulating washer 13 may be of the order of from 30 pF to 50 pF. Thus, since the capacitance C of the detector is not determined by the capacitance C11 of the piezoelectric element 11 alone, the overall capacitance C of the detector may vary greatly because it is determined not only by the capacitance C11 of the piezoelectric element 11 but also by the capacitance C13 of the insulating washer 13. By the way, the electrostatic capacitance C11 of the piezoelectric element 11 is 400 pF, for example.
Moreover, the piezoelectric element 11 generates an electric charge Q in response to the stress acting on it, and the capacitance C of the acceleration detector equals to the sum of the capacitances C of the acceleration detector equals to the sum of the capacitances C11 and C13. Therefore, the output voltage V from the acceleration detector can be expressed by V=Q/(C11+C13). From this equation, it is understood that the fluctuation of the output voltage V of the acceleration detector is increased by the fluctuation of the capacitance C13 of the insulating washer 13.
Also, during assembly of the acceleration detector, the tightening rotation of the stop nut 15 on the bushing 4 of the housing 1 can cause the rotation of the acceleration transducer assembly 3 relative to the components and the housing 1 due to the friction between them. This may cause the misalignment of terminal lead 12a in the circumferential direction relative to the external output terminal 9. If such the misalignment is too large, the electrical connection between the output terminal 12 and the external output terminal 9 becomes impossible. In such case, the stop nut 15 must be loosened, the washer terminal must be turned into the correct position, and the stop nut 15 must be tightened again with a great care and hope so that no misalignment occurs this time. This results in an extended assembly time, leading to an increased cost of the acceleration detector.
Further, since the external output terminal 9 is connected through the connector 8 to an unillustrated connector extended from an external circuit (not shown), the detection characteristics of the acceleration transducer assembly 3 is disturbed as illustrated in FIG. 4 by the oscillations of the external output terminal 9, the connector terminal (not shown) of the external circuit (not shown) coupled to the external output terminal 9, the housing 1 including the connector 8 and the connector (not shown) of the external circuit (not shown) and the like.